US3828296A - Sheathed electric heater elements - Google Patents

Abstract

Sheathed electric heater elements wherein the sheathing is made of an alloy including specially proportioned amounts of nickel (about 15 percent to about 23 percent), chromium (about 17 percent to about 23 percent), silicon (about 0.3 percent to about 1.5 percent) and cerium (an effective amount up to about 0.05 percent) to assure high oxidation resistance and good corrosion resistance.

Description

United States Patent ['19] Eiselstein et al.

[ Aug. 6, 1974 SHEATHED ELECTRIC HEATER ELEMENTS [75] Inventors: Herbert Louis Eiselstein; James Crombie Hosier, both of l-lintungton, W. Va.

[73] Assignee: The International Nickel Company,

Inc., New York, NY.

22 Filed: Jan. 18, 1973 21 Appl. No.: 324,656

Related US. Application Data [63] Continuation-in-part of Ser. No. 56,977, July 21,

1970, Pat. NO. 3,729,308.

[52] US. Cl. 338/234, 338/238 [51] Int. Cl H0lc l/02 [58] Field of Search 338/234; 219/520;

[56] References Cited UNITED STATES PATENTS 3,369,209 2/1968 Edwin 338/238 Primary Examiner-E. A. Goldberg 5 7 ABSTRACT Sheathed electric heater elements wherein the sheathing is made of an alloy including specially proportioned amounts of nickel (about 15 percent to about 23 percent), chromium (about 17 percent to about 23 percent), silicon (about 0.3 percent to about 1.5 percent) and cerium (an effective amount up to about 0.05 percent) to assure high oxidation resistance and good corrosion resistance.

9 Claims, 1 Drawing Figure SHEATHED ELECTRIC HEATER ELEMENTS The present application is a continuation-in-part of US. application Ser. No. 56977, filed July 21, 1970 now US. Pat. No. 3,729,308.

This invention is directed to improved oxidation and corrosion resistant low cost iron-base alloys particularly suitable for use as electric heater element sheath- One type of electric heater element comprises a resistance conductor which is enclosed in a tubular metal sheath, with the resistance conductor embedded in and supported in spaced relation to the sheath by a densely compacted layer of refractory, heat-conducting, electrically insulating material. The resistance conductor may be a helically wound wire member and the refractory insulating material may be granular magnesium oxide (MgO).

A commercial alloy which is currently used as sheathing material is an iron-base alloy which nominally contains, by weight, 32.5 percent nickel, 21 percent chromium, and small amounts of carbon, manganese, sulfur, silicon, copper, aluminum and titanium. While this commercial alloy has performed satisfactorily in the heater sheathing application, an alloy exhibiting enhanced resistance to oxidation at lower cost would be a highly desirable material. Moreover, it would also be of considerable commercial advantage were these benefits to be obtained without adversely affecting other characteristics for which such prior art alloys are known, notably resistance to stress-corrosion cracking and good weldability. The present invention is therefore primarily directed to achieving these overall objectives.

lt has'now been discovered that specially proportioned, iron-nickel-chromium alloys containing correlated percentages of chromium, nickel, silicon and advantageously, cerium, exhibit extraordinary resistance to oxidation at elevated temperature and, in addition, are quite resistant to stress-corrosion cracking as well as being readily weldable.

It is an object of this invention to provide an ironbase alloy which is weldable and exhibits a high resistance to cyclic oxidation at elevated temperatures and good resistance to stress corrosion cracking.

It is another object of this invention to provide an electric heater element which is sheathed in an ironbase alloy characterized by excellent resistance to cyclic oxidation at elevated temperatures and good resistance to stress corrosion cracking.

Other objects and advantages will become apparent from the following description in conjunction with the drawing which depicts in cross section a sheathed electric heater element.

Generally speaking, the present invention contemplates weldable iron-base alloys composed of, by weight, from about percent to about 23 percent nickel, from about 17 percent to about 23 percent chromium, from about 0.3 percent to about 1.5 percent silicon, cerium in a small but effective amount sufficient to improve oxidation and corrosion resistance, the cerium being up to about 0.05 percent, up to about 2 percent manganese, up to about 0.15 percent carbon, up to about 0.5 percent aluminum, up to about 0.5 percent titanium, up to about 0.015 percent sulfur and the balance essentially iron. In addition it is important that the various constituents of the alloys be correlated so as to satisfy the following relationships which, upon being observed, result in high resistance to cyclic oxidation and good resistance to stress corrosion cracking, respectively:

1178 9986 Ce) 1085 Si) -49.78 Cr) 3965 Si X Ce) 304.5 Cr X Ce) 44.84

(% Si Cr) 16.65 Mn/(% Si Ce)] must be l156+ 109.1 Ni) +2690 Mn X Ce) 5.97 Mn X %Ni) 1.57 Ni %Cr) 358.5 S1 must be 500.

Where the optimum of properties would not consistently be required nickel can be lowered down to about 8 percent, e.g., about 10 percent and the cerium can be omitted. Even then it is much preferred to use, at least a small amount, e.g., 0.005 percent of cerium, for it has been found that, in conjunction with silicon, cerium counteracts the tendency for manganese to detract from oxidation resistance and resistance to stress corrosion cracking. Accordingly, these elements should be controlled such that the ratio %Mn/(%Si %Ce) has a value less than about 0.6 and, advantageously, the ratio %Ni/%Cr has a value in the range from 0.9 to 1.2. The alloys of the invention exhibit resistance to cyclic oxidation at 1,800F. equal to or superior to that of the sheathing alloys commonly used.

An advantageous composition of the alloy of the invention is composed of, by weight, about 20% nickel, about 20% chromium, about 0.6% silicon, about 0.02% cerium and the balance essentially iron except for small amounts of incidental elements and impurites including up to about 1% manganese.

In carrying the invention into practice, chromium and nickel promote resistance to oxidation, as well as resistance to general corrosion and to stress corrosion. Chromium in amounts in excess of about 23 percent has an adverse effect on workability. In general, the higher the chromium content within the specified range, the better the alloy produced, particularly with respect to oxidation resistance. Amounts of nickel beyond 23 percent are unnecessary and should the percentage fall much below 15 percent various characteristics can be adversely affected. In seeking the best combination of results a range of from 19 percent to 21 percent or 22 percent chromium and from about 18 percent to 22.5 percent nickel is quite satisfactory.

Silicon and cerium both aid in providing the required degree of oxidation and corrosion resistance. Silicon confers improved oxidation resistance; however, an amount of silicon above the range specified leads to problems in weldability, particularly hot cracking and excessive fluidity. A silicon content of from 0.6 percent or 0.7 percent to about 1 percent or 1.1 percent is deemed quite beneficial. Cerium is advantageously present in the alloys in the amount of from about 0.01 percent to about 0.05 percent. The element cerium in amounts within the specified range makes an important contribution to the oxidation resistance and to stress corrosion resistance. It is good practice to include at least 0.01 percent cerium since the alloys are then less sensitive in respect of their corrosion resistance to variations in the amounts of the other alloy constituents, particularly, nickel, chromium, silicon and manganese. Quantities of cerium in excess of the specified range lead to difficulties in forging, rolling and in weldability. Thus, a cerium range of from 0.015 percent to about 0.04 percent is preferred.

The alloys of the invention may also include relatively small amounts of other elements which can be present as impurities without substantial detrimental effect. Such elements are contained in the alloys of the additions (the latter in the form of mischmetal) were added, and last of all, calcium. The heat was then poured into molds to produce 4 inch diameter ingots. The ingots thus prepared were forged into V4-inch thick 1 flat bars and then the flat bars were cold rolled to 0.125 inch thick sheet. Sections werecut from the sheet for use as cyclic oxidation specimens and for standard U- bend stress corrosion cracking test specimens and the remaining sections of the sheet were then welded end to end to make a roll of sheet which was then cold rolled to 0.025 inch thick sheet, the gauge commonly employed in making heater element sheathing. The folqwinsT lair esthesqmm itio 9f an mhszr otal;

loys which were prepared as described above.

TABLE I Chemical Analysis of Alloys Alloy No. C Mn Fe S Si Cu Ni Cr Al Ti Ce 1 0.05 0.41 Bal. 0.009 0.90 0.02 22.14 18.62 0.31 0.02 0.044 2 0.05 1.10 Bal. 0.005 0.71 0.02 20.27 20.37 0.11 0.12 0.022 3 0.04 0.08 Bal. 0.007 0.81 0.09 20.69 19.64 0.11 0.07 0.018 4 0.09 0.36 Bal. 0.008 0.93 0.02 18.02 17.63 0.38 0.22 5 0.11 1.63 Bal. 0.008 0.84 0.02 18.20 17.19 0.022 0.14 0.027 6 0.09 0.41 Bal. 0.008 1.03 0.02 22.06 22.72 0.004 0.19 7 0.09 1.80 Bal. 0.006 0.94 0.02 22.21 23.01 0.17 0.15 0.020

Bal. halang c iron plus impurities w invention due to their unavoidable presence in the raw materials employed, or as a result of their use for one purpose or another during the preparation of the alloys. Thus, manganese in the amount of up to 2 percent, by weight, may be tolerated, but it is more advantageous to have no more than 1 percent present. Manganese in amounts in excess of the specified maximum has an adverse effect upon oxidation resistance and upon stress corrosion cracking. Up to about 0. percent by weight of carbon can be tolerated, but it is desirable that the alloy contain no more than about 0.1 percent. Excessive carbon may result in precipitation of carbides and hence, brittleness, which gives rise to problems in forming the alloy. It is quite difficult to obtain an alloy of this type which is entirely free from sulfur, and in this case up to 0.015 percent can be tolerated. Copper is also a common impurity which can be tolerated in amounts up to a maximum of 0.5 percent. Amounts of copper greater than this tend to reduce high temperatureoxidation resistance. Aluminum, titanium and calcium are used as deoxidizers in melting the alloys of the invention, and residual amounts of these elements may remain in the final alloy composition. Thus, up to about 0.5 percent each, by weight, of aluminum and titanium may be present and in some cases, small amounts of calcium have been detected.

As shown in the drawing a sheathed electric heater element comprises an electric resistance wire 11 encased in a sheath l2 and separated therefrom by cementaceous insulating medium 13.

For the purpose of giving those skilled in the art a better appreciation of the advantages of the invention, the following illustrative examples are given.

EXAMPLE A number of heats of the alloys of the invention were prepared by charging appropriate amounts of iron, nickel and chromium into an air induction furnace and melting down the charge. Just before tapping the heat, the required silicon, aluminum, titanium and cerium The tensile properties and hardness at room temperature of the alloys of Table 1 are set forth in the following Table 11. v v we V TABLE I1 MECHANICAL PROPERTIES AT ROOM TEMPERATURE Material: Forged 9/16 inch square Bar Heat Treatment: 1800F./1 hour, air cooled The mechanical properties indicated in the above Table 11 for the alloys of the invention are quite comparable to the properties of the commonly employed prior art sheathing alloy referred to above herein. Such an alloy is produced to meet a sheet specification wherein the minimum tensile strength is 75,000 psi, the minimum yield strength (0.2 percent offset) is 30,000 psi, the minimum elongation is 30 percent and the maximum hardness is R,,.

In the following Table 111, the depth of the oxidation attack on alloy sheet having a thickness of 0.125 inch and the weight loss sustained thereby is set forth under cyclic oxidation conditions wherein a test cycle was employed in which samples were heated to a temperature of 1,800F. in a furnace in air for 15 minutes, followed by 5 minutes cooling in air outside the furnace, such cycles being repeated for a period of 1,000 hours.

TABLE I11 OXIDATION ATTACK Alloy No. Depth of Attack Weight Loss mg/cm It will be observed from the above table that the alloys of the present invention, after 1,000 hours of test, have sustained relatively slight damage from oxidation attack. A commercial alloy of the composition described above and tested under the same conditions sustained oxidation damage to a depth of about 0.008 inch and a weight loss of over 120 mg/cm Alloys which sustain a weight loss of up to 60 mg/cm in this oxidation test are considered to have exhibited high resistance to cyclic oxidation.

The alloys of Table l were also tested for resistance to stress corrosion cracking. Standard restrained U- bend stress corrosion cracking test specimens were prepared from alloy pieces which had the'dimensions 6 inches X 0.5 inch X 0.125 inch. The restrained test specimens were immersed in a boiling concentrated (45 percent) magnesium chloride solution and were periodically examined for cracking. When the tests were terminated after 720 hours (30 days) none of the alloy specimens representing the alloys of Table 1 had failed whereas the commercial alloy mentioned above failed in just over 300 hours. It is considered that alloys subjected to this test which survive 500 hours have exhibited good resistance to stress corrosion cracking. in this connection it is most advantageous that the ratio of nickel to chromium be at least unity, the silicon content should be 1 percent or more and the manganese should not exceed 0.5 percent.

The alloys of Table 1 were further tested for weldability by preparing autogenous TlG welds and examining the welds for cracking. The weldability of the alloys ranged from good to excellent.

A heat of commercial size has been produced having mechanical properties comparable to those set forth in Table 11. This heat was made in a furnace having a nominal capacity of 5,000 pounds. The heat produced weighed roughly 5,500 pounds and had the following composition expressed in weight per cent:

Nickel 20.32 Phosphorus 0.010 Chromium 19.94 Aluminum 0.17 Silicon 0.60 Titanium 0.17 Cerium 0.022 Calcium 0.001 Manganese 0.31 Copper 0.03 Carbon 0.04 lron Balance Sulfur 0.007

After forging, the heat was rolled to hot band l/4-inch thick, and thereafter cold rolled to the desired gauge of 0.090 inch. Cyclic oxidation and stress corrosion cracking test specimens were prepared and tested. The alloy exhibited a weight loss of only 3 mg/cm after 1,000 hours of the cyclic oxidation test and standard U-bend specimens for the stress corrosion cracking test were exposed to test conditions for 720 hours without failure. Accordingly, this alloy clearly exhibits high resistance to cyclic oxidation and good resistance to stress corrosion cracking.

The alloys of the present invention also exhibit a resistance to general corrosion similar to that of Type 304 stainless steel. This property is of some importance in view of the fact that in normal use as sheathing alloys, they are often exposed to contact with various solutions at elevated temperature.

There has thus been disclosed a family of alloys containing about 8 percent to about 23 percent nickel and having surprisingly good resistance to oxidation at elevated temperatures, though containing nickel substantially smaller amounts than has been the practice in the art, particularly in the field of sheathing alloys for electrical heater elements. These alloys may also be usefully employed as heat exchangers and process piping, carburizing fixtures and retorts and furnace components.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A sheathed electric heater element in which an electric resistance conductor is enclosed within a tubular metal sheath and supported in spaced, insulated relation thereto, the sheath being composed of an oxidation and corrosion-resistant alloy consisting essentially in per cent by weight, of from about 15 percent to about 23 percent nickel, from about 17 percent to about 23 percent chromium, from about 0.3 percent to about 1.5 percent silicon, from a small but effective amount of cerium up to about 0.05 percent to improve oxidation and corrosion resistance, up to about 2 percent manganese, up to about 0.15 percent carbon, up to about 0.5 percent aluminum, up to about 0.5 percent titanium, up to about 0.015 percent sulfur and the balance essentially iron, said alloy being characterized by exhibiting high resistance to cyclic oxidation resulting from balancing the constituents of the alloy within the ranges stated according to the following formula:

1178 -9986 Ce) 1085 Si) 49.78 Cr) 3965 Si X Ce) 304.5 Cr X Ce) 44.84 Si X Cr) 16.65 Mn/(%Si Ce)] s 60.

and good resistance to stress corrosion cracking resulting from balancing the constituents of the alloy within the ranges stated according to the following formula:

1156 109.1 Ni) +2690 Mn Ce) 5.97 Mn X Ni) 1.57 Ni X %Cr)+ 358.5 Si s 500 2. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains at least 0.01 percent cerium.

3. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains no more than 1 percent manganese.

4. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains from 18 percent to 22.5 percent nickel, from 19 percent to 22 percent chromium, from about 0.6 percent to about 1.1 percent silicon and from 0.015 percent to about 0.04 percent cerium.

5. The sheathed electric heater element of claim 2 wherein the alloy of which the sheath is composed contains about 20 percent nickel and about 20 percent chromium.

6. The sheathed electric heater element of claim 3 wherein the alloy of which the sheath is composed contains about 20 percent nickel and about 20 percent chromium.

7. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains about 20 percent nickel, about 20 percent chromium, about 0.6 percent silicon and about 0.02 percent cerium.

8. A sheathed electric heater element in which an electric resistance conductor is enclosed within a tubular metal sheath and supported in spaced, insulated relation thereto, the sheath being composed of an oxidation and corrosion resistant alloy consisting essentially,

in per cent by weight, of from about 8 percent to about 23 percent nickel, from about 17 percent to about 23 percent chromium, from about 0.3 percent to about 1.5 percent silicon, from a small but effective amount of cerium up to about 0.05 percent to improve oxidation and corrosion resistance, up to about 2 percent manganese, up to about 0.15 percent carbon, up to about 0.5 percent aluminum, up to about 0.5 percent titanium, up to about 0.015 percent sulfur. and the balance essentially iron, said alloy being characterized by exhibiting high resistance to cyclic oxidation resulting from balancing the constituents of the alloy within the ranges stated according to the following formula:

9. An electric heater element as in claim 8 wherein the alloy of the sheath contains at least about 0.01 percent cerium.

Claims (8)

1. 2. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains at least 0.01 percent cerium.
2. 3. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains no more than 1 percent manganese.
3. 4. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains from 18 percent to 22.5 percent nickel, from 19 percent to 22 percent chromium, from about 0.6 percent to about 1.1 percent silicon and from 0.015 percent to about 0.04 percent cerium.
4. 5. The sheathed electric heater element of claim 2 wherein the alloy of which the sheath is composed contains about 20 percent nickel and about 20 percent chromium.
5. 6. The sheathed electric heater element of claim 3 wherein the alloy of which the sheath is composed contains about 20 percent nickel and about 20 percent chromium.
6. 7. The sheathed electric heater element of claim 1 wherein the alloy of which the sheath is composed contains about 20 percent nickel, about 20 percent chromium, about 0.6 percent silicon and about 0.02 percent cerium.
7. 8. A sheathed electric heater element in which an electric resistance conductor is enclosed within a tubular metal sheath and supported in spaced, insulated relation thereto, the sheath being composed of an oxidation and corrosion resistant alloy consisting essentially, in per cent by weight, of from about 8 percent to about 23 percent nickel, from about 17 percent to about 23 percent chromium, from about 0.3 percent to about 1.5 percent silicon, from a small but effective amount of cerium up to about 0.05 percent to improve oxidation and corrosion resistance, up to about 2 percent manganese, up to about 0.15 percent carbon, up to about 0.5 percent aluminum, up to about 0.5 percent titanium, up to about 0.015 percent sulfur and the balance essentially iron, said alloy being characterized by exhibiting high resistance to cyclic oxidation resulting from balancing the constituents of the alloy within the ranges stated according to the following formula: 1178 - 9986 (% ce) - 1085 (% Si) - 49.78 (% Cr) + 3965 (% Si X % Ce) + 304.5 (% Cr X % Ce) + 44.84 (% Si X % Cr) +16.65 (% Mn/(% Si + % Ce)) 60.
8. 9. An electric heater element as in claim 8 wherein the alloy of the sheath contains at least about 0.01 percent cerium.

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